1. Field of the Invention
The present invention relates to a power-saving processing technique, particularly an improvement for enhancing an electricity-conservation effect.
2. Description of the Related Art
There has been known an art of keeping down a power consumption of electrical equipment. In such an art, a power consumption is restrained by reducing the frequency of a clock which is supplied to a CPU, a memory, each device or the like that configures electrical equipment, or lowering a voltage which is supplied to these. Besides, a technique of decreasing a power consumption has also been known by keeping under control a power which is supplied to them, or stopping a power supply. Such a power-consumption restraint processing is executed when a targeted apparatus element (e.g., a CPU, a memory and each device) is not used now. Or, when it can be operated with a weak power even in use (e.g., when a program which is processed in a CPU is kept idle). In addition, there has been known another art of keeping a power consumption at a proper level (e.g., refer to Japanese Patent Laid-Open No. 11-73255 specification). In that art, several power-saving modes are set which include even the power control of a peripheral device. Then, the usage situations of an execution application and the peripheral device are monitored, and a power-saving mode is chosen which can be adopted according to those usage situations.
However, in order to reduce a clock frequency, lower a voltage or bring a power supply to a halt, a fixed processing has to be executed based on the specification of an objective apparatus element. Besides, when a power supply is stopped, a certain period can be required so that an operation becomes stable. In short, a predetermined processing is necessary for keeping a power consumption lower, thus generating additional power consumption. This power consumption depends upon an apparatus element which undergoes such a power-saving processing. If a power-saving processing is given to many apparatuses, their power consumption is greater than in the case where a small number of apparatus elements are subjected to a power-saving processing. On the other hand, a power consumption during a period of time when an operation is executed in a power-saving mode is generally smaller when a lot of apparatus elements undergo a power-saving processing.
In the above described prior art, no consideration is given to such a power consumption which is spent when a power-saving processing is given to each apparatus element. As a result, the prior art has the following disadvantages. Even if switching is executed into a power-saving mode so that a power consumption can be restrained, the power consumption may be greater against the intention. Or, if a processing quantity is increased despite little change in a power consumption, the performance of equipment may get worse, such as a delay in its response.
For example, if a request to operate in an ordinary mode is made while electrical equipment is switching its own operation mode into a power-saving mode, its mode may be immediately returned to the ordinary mode. Besides, if electrical equipment returns into an ordinary mode a short period of time after it shifts into a power-saving mode, it does not operate long enough in the power-saving mode. This causes a power-saving processing to accounts for a large proportion of the operation. In this case, switching into the power-saving mode may contribute to an increase in the power consumption, contrary to the purpose.